1. Field of the Invention
Generally, the present disclosure relates to the manufacturing of semiconductor devices, and, more specifically, to various methods of forming graphene contacts on source/drain regions of FinFET devices, and the resulting FinFET device structures.
2. Description of the Related Art
In modern integrated circuits, such as microprocessors, storage devices and the like, a very large number of circuit elements, especially transistors, are provided on a restricted chip area. Transistors come in a variety of shapes and forms, e.g., planar transistors, FinFET transistors, nanowire devices, etc. The transistors are typically either NMOS (NFET) or PMOS (PFET) type devices wherein the “N” and “P” designation is based upon the type of dopants used to create the source/drain regions of the devices. So-called CMOS (Complementary Metal Oxide Semiconductor) technology or products refers to integrated circuit products that are manufactured using both NMOS and PMOS transistor devices. Irrespective of the physical configuration of the transistor device, each transistor device comprises laterally spaced apart drain and source regions that are formed in a semiconductor substrate, a gate electrode structure positioned above the substrate and between the source/drain regions, and a gate insulation layer positioned between the gate electrode and the substrate. Upon application of an appropriate control voltage to the gate electrode, a conductive channel region forms between the drain region and the source region and current flows from the source region to the drain region.
A conventional FET is a planar device. To improve the operating speed of planar FETs, and to increase the density of planar FETs on an integrated circuit product, device designers have greatly reduced the physical size of planar FETs over the past decades. More specifically, the channel length of planar FETs has been significantly decreased, which has resulted in improving the switching speed and in lowering operation currents and voltages of planar FETs. However, decreasing the channel length of a planar FET also decreases the distance between the source region and the drain region. In some cases, this decrease in the separation between the source and the drain makes it difficult to efficiently inhibit the electrical potential of the source region and the channel from being adversely affected by the electrical potential of the drain. This is sometimes referred to as a so-called short channel effect, wherein the characteristic of the planar FET as an active switch is degraded.
FIG. 1A is a perspective view of an illustrative prior art FinFET semiconductor device 10 that is formed above a semiconductor substrate 12 at an intermediate point during fabrication. In this example, the FinFET device 10 includes three illustrative fins 14, an isolation material 15 (e.g., silicon dioxide), a simplistically depicted gate structure 16, a sidewall spacer 18 and a gate cap layer 20. The gate structure 16 is typically comprised of a layer of insulating material (not separately shown), e.g., a layer of high-k insulating material or silicon dioxide, and one or more conductive material layers (e.g., metal and/or polysilicon) that serve as the gate electrode for the device 10. The gate structure 16 may be formed using either so-called “gate last” or “replacement gate” manufacturing techniques. The fins 14 have a three-dimensional configuration: a height H, a width W and an axial length L. The direction of current travel when the device 10 is operational, i.e., the gate length (GL) of the device 10, corresponds to the direction of the axial length L of the fins 14. The portions of the fins 14 covered by the gate structure 16 are the channel regions of the FinFET device 10, while the portions of the fins 14 positioned laterally outside of the spacers 18 are part of the source/drain regions of the device 10. Although not depicted, the portions of the fins 14 in the source/drain regions may have additional epi semiconductor material formed thereon.
FIG. 1B is a perspective view of yet another illustrative prior art FinFET semiconductor device 10A that is formed above a semiconductor substrate 30. In this example, the FinFET device 10A includes three illustrative fins 32, an isolation material 34, a simplistically depicted gate structure 36, a sidewall spacer 38 and a gate hard mask 40. The fins 32 have a three-dimensional configuration: a height H, a width W and an axial length L. In this example, the fins 32 are comprised of a substrate fin portion 32A and an alternative fin material portion 32B, e.g., SiGe, SiC, etc. The substrate fin portion 32A may be made of silicon, i.e., the same material as the substrate 30, and the alternative fin material portion 32B may be made of a material other than the substrate material, such as, for example, a compressively stressed silicon-germanium material for a PFET device or a tensile stressed silicon-carbide material for an NFET device. Such alternative materials are formed in an effort to increase the current carrying capabilities of the FinFET devices.
Unlike a planar FET, in a FinFET device, a channel is formed perpendicular to a surface of the semiconducting substrate so as to increase the drive current per footprint of the device. Also, in a FinFET, the improved gate control through multiple gates on a narrow, fully-depleted semiconductor fin significantly reduces the short channel effects. When an appropriate voltage is applied to the gate electrode of a FinFET device, the surfaces (and the inner portion near the surface) of the fins 14, 32, i.e., the vertically oriented sidewalls and the top upper surface of the fin (for a tri-gate device), form a surface inversion layer or a volume inversion layer that contributes to current conduction. In a FinFET device with a single fin, the “channel-width” is estimated to be about two times (2×) the vertical fin-height plus the width of the top surface of the fin, i.e., the fin width. Multiple fins can be formed in the same foot-print as that of a planar transistor device. Accordingly, for a given plot space (or foot-print), FinFETs tend to be able to generate significantly higher drive current than planar transistor devices. Additionally, the leakage current of FinFET devices after the device is turned “OFF” is significantly reduced as compared to the leakage current of planar FETs, due to the superior gate electrostatic control of the “fin” channel on FinFET devices. In short, the 3D structure of a FinFET device is a superior MOSFET structure as compared to that of a planar FET, especially in the 20 nm CMOS technology node and beyond.
Over recent years, due to the improvements in the performance of transistor devices, one limiting factor relating to the operating speed of the final integrated circuit product is no longer the individual transistor elements but the electrical performance of the complex wiring system that is formed above the device level that includes the actual semiconductor-based circuit elements. Typically, due to the large number of circuit elements and the required complex layout of modern integrated circuits, the electrical connections of the individual circuit elements or “wiring” for the circuits cannot be formed or positioned at the same level that contains the actual circuit elements such as transistors. Rather, the electrical wiring for the integrated circuit products is comprised of several metallization layers positioned over the device level. Such metallization layers generally include metal-containing lines providing the intra-level electrical connections, and also include a plurality of inter-level connections or vertical connections, which are also referred to as vias. These vertical interconnect structures comprise an appropriate metal and provide the electrical connection of the various stacked metallization layers.
Furthermore, in order to actually connect the circuit elements formed in the semiconductor material with the metallization layers, an appropriate vertical contact structure is provided, a first end of which is connected to a respective contact region of a circuit element, such as a gate electrode and/or the drain and source regions of transistors, and a second end that is connected to a respective metal line in the metallization layer by a conductive via. Such vertical contact structures are considered to be “device-level” contacts or simply “contacts” within the industry, as they contact the “device” that is formed in the silicon substrate. The contact structures may comprise contact elements or contact plugs having a generally square-like or round shape that are formed in an interlayer dielectric material, which in turn encloses and passivates the circuit elements. In other applications, the contact structures may be line-type features, e.g., source/drain contact structures.
Another problem with continued scaling of transistor devices is that the electrical resistance between the conductive device-level contacts and the transistor element is becoming a larger portion of the overall electrical resistance. Traditionally, metal silicide layers or regions are formed in the source/drain regions of a device and on the gate electrode of a device in order to reduce electrical contact resistance where contact will be made by a device level contact. The typical steps performed to form metal silicide regions are: (1) depositing a layer of refractory metal (e.g., nickel, platinum, etc.); (2) performing an initial heating process causing the refractory metal to react with underlying silicon-containing material; (3) performing an etching process to remove unreacted portions of the layer of refractory metal; and (4) performing an additional heating process to form the final phase of the metal silicide. In other cases, a metal silicide region may be formed by depositing a thin liner of metal, such as Ti or NiPt, depositing a TiN barrier on the liner and depositing a low-resistance conducting metal, such as W, on the barrier. The silicide is formed between the liner and conducting metal during the thermal process that occurs when the conducting metal is deposited. As such, no additional refractory metal stripping process is needed.
Ideally, the contact area between the metal silicide layer or region and the underlying silicon or epi semiconductor material (in the source/drain region) could simply be increased. In the case of FinFET devices that have additional epi semiconductor material formed on the fins in the source/drain regions of the device, this could theoretically be accomplished by forming the additional epi material on the fins in an un-merged condition, i.e., a situation where there is no contact between additional epi material on adjacent fins, and thereafter forming an individual metal silicide layer that wraps around each of the separated epi materials. In practice, this is a very difficult task for several reasons. First, when epi semiconductor material is grown on a fin, it is very difficult to control the thickness of the epi semiconductor material. Thus, the epi material may unintentionally be merged together, thereby preventing the formation of the wrap-around metal silicide layers. One possible solution to avoid such unintended fin merger would be to form the epi material on the fin to a very small thickness to virtually assure that unintended fin merger does not occur. The drawbacks to this approach are that such a very small volume of epi material will tend to increase the overall resistance and such a thin layer of epi material may be substantially consumed by the metal silicide material and/or damaged during the contact formation process.
The present disclosure is directed to various methods of forming graphene contacts on source/drain regions of FinFET devices, and the resulting FinFET device structures, that may solve or reduce one or more of the problems identified above.